This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Recognition memory decisions can be based on the perceived memory strength elicited by a test probe. One factor that is posited to influence memory strength is the degree of perceptual similarity between studied stimuli and test probes. Systematic manipulation of study-test perceptual similarity may provide leverage on the behavioral and neural bases of recognition decisions. In this experiment, we parametrically varied the degree of perceptual similarity between studied items and test probes to examine the impact of similarity on perceived memory strength and on the neural correlates of memory strength at retrieval. At study, shapes were encoded during performance of a size judgment task. At test, functional MRI (fMRI) measured neural responses as subjects made memory decisions about three types of test probes that varied in similarity to studied items: studied shapes, unstudied shapes that were morphs between a studied and an unstudied shape, and unstudied shapes. Subjects indicated whether they recognized the shape as old or new.